1. Field of the Invention
The present invention relates to a SAW (Surface Acoustic Wave) filter, and more particularly to a SAW filter having filter characteristics adjustable in an assembling process therefor and providing a wide passband width and a large attenuation level.
2. Description of the Related Art
In recent years, small-size communication devices such as cellular phones or the like have seen widespread use among many users. Such small-size communication devices employ many filters as frequency selecting devices. A transversal SAW filter which has an IDT (InterDigital Transducer) disposed on a surface of a piezoelectric member for selecting a frequency component based on an interaction between a surface acoustic wave propagated through the surface of the piezoelectric member and the IDT is widely used as a filter in such small-size communication devices because it is relatively inexpensive, lends itself to being reduced in size, and can easily be mass-produced.
However, the transversal SAW filter has, in principle, filter characteristics such as a passband width and an attenuation level determined by an electrode pattern of the IDT that has been adopted when it has been designed. Therefore, if the characteristics of a completed SAW filter do not satisfy desired specifications, then a different SAW filter has to be designed again. Even with such efforts, it is generally difficult for a SAW filter to achieve a wide passband and a large attenuation level outside of the passband.
Japanese laid-open patent publication No. 5-183380 (JP-A-5-183380) discloses a ladder filter having SAW resonators in which an inductor is inserted in series to the SAW resonator in the parallel arm and/or the series arm of the ladder filter. The ladder filter achieves a wide passband and a large suppression level outside of the passband by moving a resonance point and an anti-resonance point depending on the inductance value of the inserted inductor. The SAW resonator in the series arm is a SAW resonator inserted in a path interconnecting input and output terminals of the ladder filter, and the SAW resonator in the parallel arm is a SAW resonator interconnecting a node on a path interconnecting input and output terminals of the ladder filter and a common ground point.
However, because the inductor is inserted in series to the SAW resonator in the ladder filter revealed in JP-A-5-183380, the ladder filter is disadvantageous in that it suffers small degree of freedom in designing and its productivity is low since the ladder filter needs to be designed and prototyped again and again when the ladder filter is actually manufactured.